The present invention relates to a method for synchronization in a redundant communication system that includes a central participant and at least one further participant, at least one of the further participants being synchronized with the central participant, and, to this end, at least two telegrams containing identical synchronization information is transmitted by the central participant to the at least one further participant. The present invention further relates to a redundant communication system and a corresponding automation system.
Communication systems are known from the related art. Distributed communication systems, in particular, are utilized in many technical applications. Distributed communication systems are used, e.g., in automation systems based on decentralized control and drive system engineering, in which a large number of individual systems are often controlled and driven in a temporally synchronized manner. An example of a single system of this type is a drive unit, e.g., with a synchronous or asynchronous motor used to drive one of many axes that function in a manner such that they are mutually interpolating or closely interconnected. Typical fields of application of automation systems of this type based on decentralized control and drive system engineering are printing machines or machine tools, and robotic systems with a large number of conveying and operating elements harmonized with respect to time.
Communication systems of this type include at least two, but usually many more participants, which are preferably configured and/or arranged in a hierarchical structure, with one participant being configured as the central participant and the remaining participants being configured and/or arranged as further participants in the communication system. A hierarchical architecture of this type is known, e.g., as a master-slave structure with the central or main participant as the “master” or “master participant” (main station), and the further participants as “slaves” or “slave participants” (substations or secondary stations). The main participant is designed as the central participant that generates and sends control signals to the further participants. The further participants are in communication contact with the central participant to receive these control signals and to communicate further with the central participant, as necessary, and they are typically in communication contact with the other participants as well. The slave participants are usually process interfaces, such as sensors and actuators, i.e., input/output assemblies for analog and digital signals, and drives. Signal processing, with data preprocessing, must be decentralized among the slave participants to keep the quantity of data to be transmitted low. This requires that the master participant and the further slave participants communicate with each other. In this regard, three basic architectures (“topologies”) are known from the related art. They are illustrated in FIGS. 1 through 3. In FIG. 1, central participant M and further participants S1, S2, S3 are interconnected in a ring structure. A signal generated by central participant M travels around the ring and therefore passes each of the other participants S1, S2, and S3 in series. FIG. 2 shows a bus topology with a centralized bus line to which central participant M and further participants S1, S2 and S3 are connected. The signal and data transfer is accomplished via a data bus in a known manner. When the central bus line has long paths, it is common to interconnect a “repeater” R in the central bus line to amplify the signal. The third structure shown in FIG. 3 is a star architecture with a central switching element Sw (a “switch”) integrated in the connecting line. A signal generated by central participant M is relayed via switching element Sw to participant S1 or S2 or S3 specified as the receiver.
The three topologies shown in FIGS. 1 through 3 can also be part of a more complex system in which a plurality of basic architecture designs are realized in an interconnected manner. In this case, one of the central participants or a superordinate central participant has the task of generating a superordinate control signal.
Distributed communication systems are also known from the related art, with which the master function can be transferred among a plurality of participants or even among all participants. A requirement of “multi-master” systems of this type is that a plurality of participants have the functionality of a central participant and that they exercise this functionality when a defined condition exists. In this process, a participant that previously served as a further participant becomes the central participant, and the previous central participant becomes the further participant in the communication system. A possible condition for a transfer of this type can be, e.g., the absence of a control signal from the previous central participant.
The applicant currently offers a distributed communication system of this type with a ring-type structure on the market, called the SERCOS Interface® (SErial Real Time COmmunication System). This system generates and sends control signals via a central participant to further participants. The further participants are typically connected with the central participant via optical waveguides. The SERCOS interface® specifies strictly hierarchical communication. Data are exchanged in the form of data blocks, the “telegrams” or “frames”, between the controller (master) and the substations (slaves) in temporally constant cycles. The further participants and/or substations do not communicate directly with each other. In addition, data contents are specified, i.e., the significance, depiction and functionality of the transmitted data are predefined to a significant extent. With the SERCOS interface®, the master connects the controller to the ring, and a slave connects one or more substations (drives or I/O stations). A plurality of rings can be linked to one controller, with the controller being responsible for coordinating the individual rings with each other. This is not specified by the SERCOS interface®. This communication system is preferably used for the closed-loop and open-loop control of distributed motors, e.g., synchronous or asynchronous motors. The further participants in the communication system are, therefore, the control devices for the closed-loop and open-loop control of a motor. The main applications for this communication system are, in particular, drives of machine tools, printing presses, operative machines, and machines used in general automation technology. With the SERCOS interface® there are five different communication phases. The first four phases (phase 0 through phase 3) serve to initialize the participants, and the fifth phase (phase 4) is regular operation. Within one communication cycle, every substation exchanges data with the controller. Access to the ring is deterministic within collision-free transmission time slots. FIG. 4 shows a schematic depiction of the communication cycle of regular operation, i.e., communication phases 3 and 4 of the SERCOS interface®. With the SERCOS interface® there are three types of telegrams: Master Synchronization Telegrams, Acknowledge Telegrams and Master Data Telegrams. Master Synchronization Telegrams (MST) are sent out by the master participant. They contain a short data field, are used to define the communication phase and serve as the “clock”. Acknowledge Telegrams (AT) are sent by slave participants and include, e.g., actual values of a drive controlled by the particular slave participant. Master Data Telegrams (MDT) are summation (framework) telegrams that contain data fields for all slave participants. The master uses Master Data Telegrams to transmit setpoint values to each slave. During initialization, every substation is notified of the start and length of its (sub-) data field. The SERCOS interface® defines the following types of data, i.e., operating data, control and status information, and data transmitted in a non-cyclic manner. Operating data (process data) are transmitted in every cycle. Examples include setpoint values and actual values.
The length of the operating data range is parametrizable. It is established during initialization and remains constant during operation of the ring. The control information transmitted by the master participants to the slave participants, and the status information sent by the slave participants to the master participants are, e.g., release signals and “ready” messages. Data transmitted in a non-cyclic manner (service channel) include setting parameters, diagnostic data and warnings. Command sequences are also controlled via this non-cyclic transmission. As shown in the schematic depiction in FIG. 4, a communication cycle is started by the central participant sending out an MST. All communication-specific times are based on the end of this short (approx. 25 □s-long) telegram. The substations now send their Acknowledge Telegrams (AT) in succession, in their respective transmission time slots, starting with T1,i. After the last AT, the master sends the MDT, starting at T2. The next cycle begins with another MST. The time interval between two MSTs is referred to as SERCOS cycle time TSYNC. With the SERCOS interface®, communication is synchronized with the end of the MST. A synchronization telegram is generated by the central participant—preferably at equidistant intervals—and fed into the communication ring. In the closed-loop controllers, a time parameter typically links receipt of the synchronization telegram and the synchronization signal with the processing of setpoint/actual values, which results in a determination and allocation of open-loop and closed-loop parameters to the particular servo motors.
Synchronization of the participants is of prime importance. If inaccuracies occur in the synchronization of the further participants with the central participant, the processes controlled by the further participants are not carried out in a synchronized manner. For example, the printed image produced by a printing press with servomotors controlled by a conventional communication system of this type can be blurred as a result of inadequate synchronization of the motion sequences. The same applies for machine tools or other automation machines that require highly-exact synchronization of processes. In the case of machine tools, for example, faulty synchronization can result in inexact machining of a workpiece, since, e.g., individual axes (e.g., x, y and z-axes) move such that they are temporally unsynchronized.
Due to the significance of synchronization of the further participants with the central participant described above, an error in a telegram containing synchronization information sent out by the central participant is extremely problematic.
If a telegram containing synchronization information was corrupted in a communication cycle, as can be detected by evaluating the checksum, drives cannot be controlled exactly in this communication cycle, for example. It is therefore necessary to design the communication system and its protocols such that the likelihood of a telegram containing faulty or destroyed synchronization information is minimized.